Trucks, such as a vacuum truck, are used to render many types of services in hydrocarbon environments, including the removal of slop oil, sludge and gasoline. When loading, a truck, such as a vacuum truck, functions by creating a sufficient pressure (vacuum) on the tank of the truck to cause material to be pulled into the tank via the hose connected to the truck. When unloading, the procedure is reversed and material is removed from the tank either by gravity feed or because the contents of the tank are put under pressure and the material is more forcibly expelled. In either loading or unloading operations, vapors (or gases) may be given off and the vapors may become flammable when mixed with corresponding levels of oxygen found in the air. The point at which a mixture is flammable can be measured as the lower explosion limit or the "LEL" reading. A situation where a mixture of gases and oxygen measured at the "LEL" level is combined with an ignition source may result in a fire or an explosion.
One type of an ignition source results from the excessive build-up of static electricity. Static electricity may be created by fluid movement through hoses, pipes, or any container, either when loading or unloading. There are several potential locations for static electricity build-up on a vehicle which is typically a truck. For example, static electricity may build up on any of the metal protrusions or parts inside the tank which come into contact with moving fluid. In addition, static electricity may also build up on the hose connected to the truck.
To prevent excessive static electricity build-up within the tank, a grounding cable can be run from the truck chassis to the tank or vessel being emptied or filled to ground the truck chassis. To prevent the hose from being the second potential ignition location, a hand held ohmmeter has been used to check the "ohms" required to ground the hose while one or more hoses are connected together and are being used to load or unload the truck. Either an internal wire on the hose or a separate wire, run externally and connected to the hose at coupled connection points, provides a ground flow path.
However, it has recently been discovered that the systems described above may not be as efficient as originally thought. For example, it has been discovered that worker error can result in a grounding connection which has not been properly established. Although, a meter can be used to check that a proper grounding connection has been established, such a system is dependent on the truck operator. For example, if the truck operator does not read or use an ohmmeter properly, a faulty grounding connection may not be discovered. Also, even after grounding connections are made and confirmed, it is possible that the initiation of fluid movement (e.g., vacuuming) operations may cause the grounding connection to be lost or intermittently interrupted while the truck is in use as a result of equipment or environment changes, an improper set-up or improper operation of the truck. If the ground connection is lost, or interrupted, static electricity may begin to build-up.
Other potential types of ignition sources are the truck motor and the truck electrical system. In addition, the ignition sources which are in close proximity to hydrocarbon fumes in the work area itself (for example, the truck motors and electrical systems of other equipment which may be located near the vacuum truck) may cause the mixture of gases and oxygen to ignite. Presently, a vacuum truck operator may use his/her sense of smell or a hand held "LEL" meter to alert him/her of an LEL danger. These systems function by allowing the truck operator enough time to disable the operation of the truck before an ignition source ignites the gas and oxygen mixture.